[SPARK-36939][PYTHON][DOCS] Add orphan migration page into list in PySpark documentation
[SPARK-36939][PYTHON][DOCS] Add orphan migration page into list in PySpark documentation

### What changes were proposed in this pull request?

This PR fixes the warning below during PySpark documentation build:

```
checking consistency... /.../spark/python/docs/source/migration_guide/pyspark_3.2_to_3.3.rst: WARNING: document isn't included in any toctree
done
```

SPARK-36618 added a new migration guide page but that's mistakenly not added to `spark/python/docs/source/migration_guideindex.rst` resulting in not being shown.

### Why are the changes needed?

To show the migration guides to end users.

### Does this PR introduce _any_ user-facing change?

It's not yet released but we should better backport to branch-3.2.
It's a followup of the new page in PySpark documentation (branch-3.2).

### How was this patch tested?

Manually built the docs

Closes #34195 from HyukjinKwon/SPARK-36939.

Authored-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>
(cherry picked from commit 0902b47a7529d036afdaf181d96abdb2fbfb8d77)
Signed-off-by: Hyukjin Kwon <2680965ad5aae29275c8fb10150cb10f5554e78b@apache.org>

milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark,milliman/spark